1. Field of the Invention
The invention relates generally to methods and systems for tracking movement of storage media, and more specifically to methods and systems for tracking storage media with embedded optical tracking features to sense linear and lateral motion of the media.
2. Description of the Related Art
Digital tape-recording remains a viable solution for the storage of large amounts of data. Conventionally, at least two approaches are employed for recording digital information onto magnetic recording tape. One approach calls for moving a magnetic tape past a rotating head structure that reads and writes user information from discontinuous transverse tracks. Interactive servo systems are typically employed to synchronize rotation of the head structure with travel of the tape. Another approach is to draw the tape across, a non-rotating head at a considerable linear velocity. This approach is sometimes referred to as linear “streaming” tape recording and playback.
Increased data storage capacity and retrieval performance is desired of all commercially viable mass storage devices and media In the case of linear tape recording, a popular trend is toward multi head, multi-channel fixed head structures with narrowed recording gaps and data track widths so that many linear data tracks may be achieved on a tape medium of a predetermined width, such as one-half inch width tape. To increase the storage density for a given cartridge size, the bits on the tape may be written to smaller areas and on a plurality of parallel longitudinal tracks. As more tracks are recorded on the tape, each track becomes increasingly narrow. As the tracks become more narrow, the tape becomes,more susceptible to errors caused from the tape shifting up or down (often referred to as lateral tape motion or “LTM”) in a direction perpendicular to the tape travel path as the tape passes by the magnetic head. In order to maintain proper alignment of the head with the data tracks on the tape, the tape is generally mechanically constrained to minimize lateral tape motion and data retrieval errors.
Lateral tape motion is generally defined as the peak-to-peak distance of the undesirable movement (in-plane) of the tape perpendicular to its prescribed longitudinal direction of motion past a read/write head. Lateral tape motion and the ability to compensate for lateral tape motion is a major limiting factor in determining the minimum width of a track and the minimum spacing between tracks on the tape. Thus, as lateral tape motion is reduced, more tracks may be stored on the tape and the tape density increases accordingly.
Tape substrates are also being made thinner to increase data storage for a given cartridge size. The thinner tape allows more tape to be contained within the same size diameter reel packages, thereby increasing the data storage of the cartridge. Thinner tapes, however, are generally less rigid making them more susceptible to lateral tape motion errors.
One approach to minimize lateral tape motion tracking errors is to provide a multi-roller tape guide structure, such as the type described in commonly assigned U.S. Pat. No. 5,414,585, entitled “Rotating Tape Edge Guide,” the disclosure thereof being incorporated herein by reference in its entirety. Such an approach has provided a viable “open loop” solution to lateral tape motion, i.e., control of lateral tape motion without the use of feedback. The advent of new head technologies, such as magneto-resistive read heads, and new higher coercivity recording media, data track widths have become very small, and many additional data tracks may be defined on the tape. Unfortunately, lateral tape motion remains as a limiting factor, and at certain data track width dimensions and data track densities, it is not possible to follow the tape accurately enough to provide reliable performance during reading and writing operations.
Several “closed loop” solutions have been developed to maintain alignment of a read/write head with data tracks and to minimize lateral tape motion tracking errors, including the use of magnetic servo tracks positioned on a the tape. Servo tracks allow for increased tracking abilities through servo track feedback mechanisms and the like. These methods, however, have not been able to keep pace with the increased data capacity desired for magnetic tape storage media, including increasingly narrow data tracks and thinner storage media. A need exists therefore for an increased ability to track storage media, including lateral tape motion, and allow for increased data storage capabilities.